Abstract: Cytotoxic biotherapeutic agents effective for treating certain types of cancer in humans are provided which comprise the TP-3 murine monoclonal antibody chemically conjugated to pokeweed antiviral protein (PAP). The invention further provides a method which utilizes the disclosed cytotoxic biotherapeutic agents to systemically treat cancer patients. With slight modifications the method of the present invention should be generally applicable to preparation and use of other cytotoxic biotherapeutic agents using chemical or recombinant derivatives of the TP-3 or TP-1 antibodies or PAP toxin. The invention is applicable to cancer patients who express the p80 antigen recognized by the TP-1/TP-3 antibodies either on the surface of their tumor cells or on the tumor blood vessels.